If a contrast‑enhanced CT of the abdomen shows liver lesions with a differential diagnosis of metastatic disease, which liver MRI protocol should be ordered?

MRI Protocol for Suspected Liver Metastases

Order an MRI abdomen with intravenous contrast using a hepatobiliary agent (gadoxetate disodium/Eovist) that includes dynamic multiphase imaging, diffusion-weighted imaging (DWI), and hepatobiliary phase (HBP) imaging at approximately 20 minutes post-injection.

Rationale for This Protocol

The ACR Appropriateness Criteria specifically recommend MRI with IV contrast for suspected liver metastases, as it provides superior lesion detection and characterization compared to CT 1. MRI has multiple critical advantages over CT including superior contrast resolution, multiphase imaging with every contrast-enhanced protocol, the ability to obtain diffusion-weighted imaging, and availability of hepatocyte-specific contrast agents 1.

Essential Protocol Components

Dynamic Multiphase Imaging

• Include arterial phase, portal venous phase (PVP), and delayed phases 1
• Multiphase imaging is mandatory because metastases behave variably—up to 59% of hyperenhancing metastases can be isodense or hypodense to liver parenchyma on either arterial or portal venous phases 1
• The patterns of enhancement on contrast-enhanced MRI are essential for characterization of hepatic lesions 1

Diffusion-Weighted Imaging (DWI)

• DWI is mandatory, especially for detection of sub-centimeter metastases 2, 3
• The combination of DWI and hepatobiliary phase allows correct classification of lesions as benign or malignant in 91% of cases and exact characterization in 85% of cases 1
• DWI is among the most sensitive sequences for liver metastasis detection 3

Hepatobiliary Phase (HBP) Imaging

• Use gadoxetate disodium (Eovist/Primovist) as the contrast agent of choice 1
• Gadoxetate provides hepatobiliary phase imaging at approximately 20 minutes post-injection, which is practical for clinical workflow 1, 4
• The hepatobiliary phase of gadoxetate correctly identified 69% to 87% of lesions in rigorous studies comparing imaging with histopathologic findings 1
• Metastatic lesions appear hypointense against the bright enhancing normal liver parenchyma on HBP, providing high tumor-to-lesion contrast 1, 4
• Gadoxetate-enhanced MRI demonstrates improved sensitivity for detecting focal liver lesions, particularly lesions <1 cm 1

Additional Standard Sequences

• T2-weighted sequences with fat suppression 2
• In- and out-of-phase gradient echo T1-weighted imaging 2

Key Advantages Over CT

• MRI established a definitive diagnosis in 95% of liver lesions, significantly higher than contrast-enhanced CT 1
• Only 1.5% of patients with MRI require recommendation for further imaging compared to 10% with CT 1
• MRI is particularly valuable when CT, ultrasound, and PET-CT are normal but clinical suspicion for metastasis remains high 3

Important Caveats

Disadvantages to Consider

• MRI examinations take 15-20 minutes compared to <5 minutes for CT 1
• Susceptibility to motion artifact related to the length of individual sequences 1
• Gadoxetate can induce transient arterial phase motion, potentially obscuring arterial phase hyperenhancement 5

Tumor-Specific Considerations

• For neuroendocrine tumors and renal cell carcinoma where lesions are predominantly hyperenhancing, arterial phase imaging is particularly important 1
• For breast cancer metastases, arterial phase has limited value as only 10-26% are hyperenhancing 1

Current Limitations

• Low sensitivity for metastases smaller than 3 mm 3
• Some hemangiomas can be hypoenhancing at venous and transitional phases and hypointense at HBP, potentially mimicking metastases 5

Alternative Consideration

If gadoxetate is unavailable or contraindicated, MRI with extracellular contrast agent (ECA) including dynamic phases and DWI is an acceptable alternative 6, though it lacks the added diagnostic value of hepatobiliary phase imaging for distinguishing hepatocyte-containing from hepatocyte-deficient lesions 1.